Low Pressure Fuel System Diagnosis

The Bosch P7100 inline fuel injection pump is very sensitive to fuel supply pressure variations. If this pressure is low or pulsating excessively, it may cause:
- Low rpm miss/instability
- White smoke
- Hard starting
- Engine dies at idle (especially when hot)
- Low power
- Fuel filter cracking (caused by high pressure spikes with restricted overflow valve or fuel return line)

If the preceding symptoms/conditions exist:

1. Check fuel shutoff solenoid adjustment. Improper adjustment could affect fuel delivery in the injection pump. Refer to Fuel Shutdown (Shutoff) Solenoid Removal/installation for adjustment procedures.
2. Visibly check the fuel supply line and fuel return line for kinks, bends or any other damage that would restrict fuel flow.
3. In addition to contaminants or air, restrictions in the fuel supply to the injection pump caused by a clogged or dirty fuel filter can cause problems. A restricted pre-filter screen within the fuel heater may also have the same results. Be sure to clean the pre-filter each time the fuel filter is replaced. Refer to Fuel Heater Removal/installation for pre-filter service procedures.
4. Check fuel transfer (lift) pump supply pressure to the injection pump. Refer to the Fuel Transfer Pump Pressure/Capacity Test.
a. Normal pressures are 17-22 psi at idle and 25-30 psi at rated rpm (2500 rpm) with no load.
b. If supply pressure is low, restrict the fuel return line (pinch off fuel return hose). If pressure returns back to normal or above, the overflow valve must be replaced. Refer to the Fuel Transfer Pump Pressure/Capacity Test.
c. If fuel supply pressure remains low with the fuel return line pinched, the fuel transfer pump must be replaced. Refer to the Fuel Transfer Pump Pressure/Capacity Test.
d. High pressure at idle speed indicates a restriction. Make sure the overflow valve has been installed at the fuel return line (pump outlet) and not at the fuel supply line (pump inlet).
e. Fuel pressure must be checked after each corrective action. Other possible causes can be found in the System Diagnosis section of Engine.

Whenever the fuel injection pump is being replaced or removed for calibration, the overflow valve must stay with the pump. Make sure a new overflow valve is used with a new injection pump and the old (original) overflow valve is returned to the authorized repair facility with the old injection pump.